Among firms inspected were Sweden's Scania and Volvo, Germany's Daimler and MAN, and Iveco in Italy.
The commission said it had "reason to believe that the companies concerned may have violated EU antitrust rules that prohibit cartels and restrictive business practices and/or the abuse of a dominant market position".
The European Union's executive branch did not reveal the names of the companies, following standard practice.
But Swedish company Scania confirmed it was among the companies investigated by the European Commission for "inappropriate exchange of information" and vowed to cooperate fully with the probe.
Scania's head office in Södertalje as well as the head offices of its sales firms in the Netherlands, Belgium and Luxembourg were probed, said spokesman company Hans-Åke Danielsson.
"They have visited us today around lunch to get hold of our archives and so on as they do as usual," he said. "It was not a dawn raid but rather an afternoon tea raid," he laughed.
Volvo also confirmed it was involved. Spokesman Mårten Wikforss said "they are also examining our subsidiaries." The Swedish giant owns Volvo trucks as well as Renault Trucks, Mack and UD Trucks.
German heavy truck maker and engineering group MAN said its headquarters had been searched and that it was cooperating with investigators.
"MAN has pledged its full cooperation to the European Union's antitrust authorities in the antitrust proceedings against several European truck manufacturers," a statement said.
MAN added that it "does not tolerate any breaches of compliance" and that it "does not currently expect customers to have suffered prejudice."
Officials at rival Daimler, the world's largest maker of heavy trucks, also confirmed it had been probed as did Iveco.
In September, British watchdog the Office of Fair Trading (OFT) said it had launched a probe into alleged price-fixing by major European truckmakers, including German giants Daimler and MAN, resulting in the arrest of one person.
British offices of Mercedes-Benz, owned by German vehicle giant Daimler, were also raided and MAN as well as Scania and Volvo said they had been asked to provide the OFT with information regarding the probe.
Unannounced EU inspections are a preliminary step and do not mean that the companies are guilty of anti-competitive behaviour, the commission said in a statement.
Fines can be as high as 10 percent of turnover.
Commission officials were accompanied by their counterparts from national competition authorities in the inspections.
There is no legal deadline to complete the investigation and the duration depends on several factors, including the complexity of each case and the extent to which companies cooperate with investigators.